Modal instability control in fiber lasers

Abstract

Fiber lasers and methods are provided, in which the modal instability threshold is raised to provide more laser power. Fiber lasers comprise an active optical fiber having at least one absorption peak wavelength (λpeak) and capable of supporting more than a fundamental mode during operation, and a plurality of pump diodes connected to deliver radiation emitted thereby into the optical fiber. At least one of the pump diodes is a wavelength-locked (WL) diode and at least one of the pump diodes is configured to deliver radiation at at least λ≠λ(not necessarily the same diode(s)). The pump diodes may comprise any of WL diode(s) at λ≠λpeak, WL diode(s) at λ=λpeak and non-WL diode(s). Pumping radiation off the fiber's absorption peak increases the modal instability threshold, most likely by reducing the temperature gradient in the active fiber at the fiber pump entrance point and along the fiber.

What is claimed is: 
     
       1. A fiber laser comprising:
 an active optical fiber having at least one absorption peak wavelength (λ peak ), capable of supporting more than a fundamental mode during operation, and 
 a plurality of pump diodes connected to deliver radiation emitted thereby into the optical fiber, wherein at least one of the pump diodes is a wavelength-locked (WL) diode, wherein at least one of the pump diodes is a non-WL diode, having different center wavelengths and wherein at least one of the pump diodes is configured to deliver radiation at at least  peak , 
 wherein the fiber laser is configured to operate at an output power higher than 400 W, and 
 wherein, in operation, a modal instability (MI) threshold of the fiber laser is higher than a base MI threshold of the active optical fiber when pumped only by WL diodes at λ peak . 
 
     
     
       2. The fiber laser of  claim 1 , wherein the active optical fiber comprises at least one of: an all-glass LMA (large mode area) type optical fiber with a core diameter >15 μm, a LMA fiber with coiling for mode stripping, a photonic crystal active fiber and a photonic bandgap structure fiber. 
     
     
       3. The fiber laser of  claim 1 , wherein the output power is higher than 1000 W. 
     
     
       4. The fiber laser of  claim 1 , wherein the pump diodes comprise at least one WL diode at λ=λ peak . 
     
     
       5. The fiber laser of  claim 1 , wherein the pump diodes comprise at least one WL diode at λ≠λ peak . 
     
     
       6. The fiber laser of  claim 1 , wherein the at least one non-WL diode has a wavelength range that includes λ peak . 
     
     
       7. The fiber laser of  claim 1 , wherein the at least one non-WL diode has a wavelength range that does not include λ peak . 
     
     
       8. The fiber laser of  claim 1 , wherein the pump diodes comprise at least one WL diode at λ≠λ peak . 
     
     
       9. The fiber laser of  claim 8 , wherein all the pump diodes are WL diodes. 
     
     
       10. The fiber laser of  claim 1 , further comprising a control module configured to control a temperature of at least one of the pump diodes. 
     
     
       11. The fiber laser of  claim 10 , wherein the control module is further configured to tune the temperature to maximize a power emitted by the fiber laser. 
     
     
       12. The fiber laser of  claim 1 , further comprising a control module configured to control a current delivered to at least one of the pump diodes. 
     
     
       13. The fiber laser of  claim 12 , wherein the control module is further configured to tune the current to maximize a power emitted by the fiber laser. 
     
     
       14. A method comprising:
 connecting a plurality of pump diodes to deliver radiation emitted thereby into an active optical fiber having at least one absorption peak wavelength (λ peak ) and capable of supporting more than a fundamental mode during operation, wherein at least one of the pump diodes is a wavelength-locked (WL) diode, wherein at least one of the pump diodes is a non-WL diode having different center wavelengths, and 
 configuring at least one of the pump diodes to deliver radiation at λ≠λ peak , 
 wherein the active optical fiber is configured to operate at an output power higher than 400 W, and, in operation, a modal instability (MI) threshold of the active optical fiber is higher than a base MI threshold of the active optical fiber when pumped only by WL diodes at λ peak . 
 
     
     
       15. The method of  claim 14 , further comprising using at least one WL diode at λ=λ peak . 
     
     
       16. The method of  claim 14 , further comprising using at least one WL diode at λ≠λ peak . 
     
     
       17. The method of  claim 15 , further comprising configuring the at least one non-WL diode to have a wavelength range than includes λ peak . 
     
     
       18. The method of  claim 15 , further comprising configuring the at least one non-WL diode to have a wavelength range than does not include λ peak .